Electrolytic production of ammonium persulfate solutions



March 18, 1952 w. s. WOOD ETAL 2,589,932

ELECTROLYTIC PRODUCTION OF AMMONIUM PER-SULFATE SOLUTIONS Filed May 8,1948 M1. LIAM sr/m/z E) W000 GEORGE C1. E/V/VETT #05527 LESL/E HULL/7N0Patented Mar. 18, 1952 ELECTROLYTIC PRODUCTION OF AMMO- NIUlW PERSULFATESOLUTIONS William Stanley Wood, Harpenden, and George Clennett andHubert Leslie Hulland, Luton, England, assignors to La Porte ChemicalsLimited, Luton, England, a British company Application May 8, 1948,Serial No. 25,984 In Great Britain May 12, 1947 2 Claims.

This invention relates to a process for the manufacture of acidsolutions of ammonium persulphate by electrolysis using electrolyticcells having a-stationary catholyte. Such solutions can be hydrolysedand distilled to yield hydrogen peroxide or treated to give ammoniumpersulphate crystals.

In the electrolytic conversion of acid solutions of ammonium sulphate toammonium persulphate it is usual to surround the cathode with adiaphragm to prevent the ammonium persulphate formed being decomposedand to obtain the maximum current efiiciency. Such diaphragms are madeof .porous ceramic material, of asbestos, of microporous ebonite, orother suitable material. It is possible in this electrolysis to usecells having separate chambers for anolyte and catholyte both of whichflow continuously through the cell. With such an arrangement thecatholyte is always acid in reaction but this involves controlling andhandling two separate flows of electrolyte through the cell withconsequent multiplicity of plant and controls. It is thereforeadvantageous to employ a single chamber cell in which the catholyte isstationary while only the anolyte is discharged continuously.

In the type of process using a stationary catholyte .a porous ceramicdiaphragm is used and, as stated above, the anolyte flows continuouslythrough the cell, eventually being discharged as an acid solution ofammonium persulphate, while the catholyte remains in the diaphragmchamber and is therefore stationary. The diaphragms heretofore used havebeen very loosely fitting so that there is always a considerable volumeof stationary catholyte in the diaphragm. As the electrolysis progressesthe concentration of ammonium ions in this stationary cat'holyteincreases and this increase in ammonium ions eventually results in analkaline condition in the catholyte. This alkaline catholyte has anincreased electrical resistance thus increasing the resistance. Thisproblem of attack cannot be solved by altering the composition of theceramic alkaline solutions.

material as it has not yet been found possible to manufacture ceramicdiaphragm material which is simultaneously resistant to both acid andThis increased resistance of catholyte and diaphragm causes an increasein the potential difference across the cell leading to an increasedconsumption of electrical energy per potential difference across thecell. Also the alkaline condition of the catholyte results in? theprecipitation of the oxides of the metallic impurities present in thesolution. This precipitation occurs in the body of the diaphragm at thealkali-acid interface and "results in the blocking up of the pores ofthe diaphragm with a consequent increase in the resistance of thediaphragm. A further disadvantage resulting from this alkaline catholyteis that this alkaline solution attacks the ceramic diaphragm materialcausing it to break down so that the pores beu come blocked whichagain'leads toan increase in temperature.

unit of ammonium persulphate produced, as well as to an increase in theamount of cooling re quired to maintain the cell at its eificientworking A further consequence is the loss of ammonia from the systemowing to its evolution from the alkaline catholyte.

I It has now been found according to this invention that the abovedisadvantages of using ceramic diaphragms with a stationary catholytecan be obviated if the liquid within the diaphragm is always maintainedacid in reaction and if a closely spaced porous ceramic diaphragm isused.

Accordingly, the present invention provides a process for themanufacture of acid solutions of ammonium persulphate by theelectrolysis of acid solutions of ammonium sulphate using anelectrolytic cell having a stationary catholyte 'whereinthe catholyte imaintained acid in reaction throughout the electrolysis and wherein thecathode is surrounded by a closely spaced porous ceramic diaphragm.

Preferably platinum anodes and graphite cathodes are employed. A cellsuitable for carrying out the process of the present invention is shownin the accompanying drawings in which Fig. 1 shows a longitudinalsectional view of the cell and Fig. 2 shows a plan view of part of thecell.

Referring to the drawings, the cell I contains a plurality of cathodes 2screwed into a bar 3 of the cell, each cathode being surrounded by aclosely fitting porous ceramic diaphragm 4. The anodes 5 are disposed inthe anolyte 6 the level of which is indicated at I. The anolyte solutionis led into the cell I through an inlet pipe 8 and discharges through anoverflow pipe 9.

- It has been found that the best results are obtained when the distancebetween the cathode and inner surface of the diaphragm does-not exceed 3mm. at any point.

In order to achieve the desired conditions of continuous acidity in thediaphragm coupled with a low potential difference across the celltogcther with maximum energy efficiency it was found. that the leakageof electrolyte through the porous diaphragm-material had tobe withincertain limits in order that there was sufficient flow of acidelectrolyte into the diaphragm to maintain the catholyte acid inreaction. The factors which affect this leakage are the permeability ofthe ceramic diaphragm material and the depth of electrolyte outside thediaphragm. The depth of liquid outside the dia-- phragm when it isplaced in the cell is referred to herein as the working level of thediaphragm. An increase in the permeability of the ceramic material or anincrease in the working level of the diaphragm with fixed permeabilityboth lead to an increasedleaka'geofelectrolyte through the diaphragm,and these two factors must be varied to obtainthe desired leakage. Wemeasure the permeability of 4 the diaphragm as follows: The leakage isexpressed as the amount of water which can flow to air through unit areaof the diaphragm per minute.

It was found that the limits of leakage for a suitable porous ceramicdiaphragm lay between I and mls. of water per sq. dm. per minute when itis filled to its working level with water.

In order to achieve the desired electrolysis conditions it was alsofound preferable to limit the current density to a figure between 3 and12 amps per sq. dm. at the cathode.

A. preferred embodiment of this invention for the manufacture of acidsolutions of ammonium persulphate comprises electrolysing and acid soofthe diaphragm is between 10 and 40. mls. per

minute when filled to the working level (measured as hereinbeforedefined).

Under these conditions the stationary catholyte remains continuouslyacid and ammonium persulphate solutions are produced with an improvedenergy efiiciency, no ammonia is lost from the system and the conditionand life of the diaphragm are greatly improved. Also such an arrangementof closely spaced ceramic diaphragms is very economical in space for thegeneral electrode assembly which is capable of easy multiplication forthe construction of large units.

As indicated above the acid solution of ammonium persulphate obtainedmay be hydrolysed and distilled to give hydrogen peroxide or it may beconverted into ammonium persulphate crystals in ways well known to thoseskilled in the art.

As mentioned above, a number of advantages result from the employment ofa catholyte which is continuously acid in reaction. Thus, the ohmicresistance of catholyte and diaphragm, and hence the potentialdifference across the cell, is reduced. A high current efficiency isobtained together with a high yield per unit of electrical energy. Thereis no alkali build up in the catholyte and hence no blocking up of thepores of the diaphragm with consequent disinteg'ration- The followingexample illustrates how the process of the invention may be carried intoeffect:

An acid solution of ammonium sulphate having the following composition:200 gms./litre (NH4)2SO4- and 300 gms./litre H2SO4, was electrolysedi'na cell having platinum anodes and a.

carbon cathode surrounded by a porous ceramic diaphragm, 36 mm. outerdiameter, 1.5 mm. wall thickness, having a leakage to air of 80 cc./min.of water when filled with water to its working level of 50 cms. and thedistance between the ceramic diaphragm and the carbon cathode was 2 mm.The current was 30 amps on the cathode, i. e., about 6 amps per sq. dm.Throughout the electrolysis it was found that the catholyte continuouslyshowed an acid reaction. The potential difference across the cell was4.9 volts. The energy efiiciency was 0.85 kWh/lb. of ammoniumpersulphate when the concentration of the ammonium persulphate solutionproduced was 260 gms./litre. After the cell had been runningcontinuously for 12 months the diaphragm was strong and in goodcondition and the original leakage was unimpaired showing that there hadbeen no blockage of the pores.

. -We claim:

1. In the manufacture of solutions of. ammonium persulphate byelectrolysis carried on in a diaphragm cell wherein a substantiallystationary body of catholyte is maintained on the cathode side of saiddiaphragm with the diaphragm spaced from said cathode a substantialdistance butnot exceeding 3 mm., the improvement which comprisessubjecting an acid solution of ammonium sulphate as anolyte toelectrolysis in said cell while causing a leakage of said anolytethrough said diaphragm to the catholyte side thereof sufficient tomaintain the oatholyte acid in reaction during the course of theelectrolysis, and maintaining a current density of between 3 and 12amps. per sq. dm. of the immersed surface of said cathode.

2. In the manufacture of solutions of ammonium persulphate byelectrolysis carried on in a porous ceramic diaphram cell wherein asubstantially stationary body of catholyte is maintained on the cathodeside of said diaphragm and the cathode is spaced from the diaphragm asubstantial distance but not exceeding 3 mm., the improvement whichcomprises subjecting an acid solution of ammonium sulphate as anolyte toelectrolysis in said cell at a current density on the cathode of between3 and 12 amps. per sq. dm. while causing a leakage of said anolytethrough said diaphragm to the catholyte side thereof during the courseof the electrolysis equivalent to a leakage of water to air of from 10to 40 mls. per sq. dm. per minute when a diaphragm of like porosity isfilled to the working level with water.

WILLIAM STANLEY l/VOOD. GEORGE CLENNETT. HUBERT LESLIE HULLAND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,059,809 Adolph et al Apr. 22,1913 1,907,812 Honsberg May 9, 1933 2,094,384 Tucker et a1 Sept.28,1937- 2,281,090 Salleras Apr. 28, 1942 2,349,998 Trinius May 30, 19442,392,868 Stuart Jan. 15,v 1946 FOREIGN PATENTS Number Country Date508,524 Great Britain July 3, 1939 510,429. 1 Great Britain Aug. 1, 1939

1. IN THE MANUFACTURE OF SOLUTIONS OF AMMONIUM PERSULPHATE BYELECTROLYSIS CARRIED ON IN A DIAPHRAGM CELL WHEREIN A SUBSTANTIALLYSTATIONARY BODY OF CATHOLYTE IS MAINTAINED ON THE CATHODE SIDE OF SAIDDIAPHRAGM WITH THE DIAPHRAGM SPACED FROM SAID CATHODE A SUBSTANTIALDISTANCE BUT NOT EXCEEDING 3 MM., THE IMPROVEMENT WHICH COMPRISESSUBJECTING AN ACID SOLUTION OF AMMONIUM SULPHATE AS ANOLYTE TOELECTROLYSIS IN SAID CELL WHILE CAUSING A LEAKAGE OF SAID ANOLYTETHROUGH SAID DIAPHRAGM TO THE CATHOLYTE SIDE THEREOF SUFFICIENT TOMAINTAIN THE CATHOLYTE ACID IN REACTION DURING THE COURSE OF THEELECTROLYTSIS, AND MAINTAINING A CURRENT DENSITY OF BETWEEN 3 AND 12AMPS. PER SQ. DM. OF THE IMMERSED SURFACE OF SAID CATHODE.